I've just purchased a set of 3 outdoor low voltage halogen lights to provide
some safety illumination for our somewhat gloomy path.

Obviously I don't want these to be lit all the time so I looked for a timer
switch. I found one that worked with inductive loads as the low voltage
halogens are run from a transformer.

This transformer has no useful information on it that I can see. It only
says that its output is 12V, 30W which is exactly what I'd expect as each of
the lights in the set of 3 is a 12V 10W halogen.

The timer switch says it will switch 13A resistive or 5A inductive loads.
I'm pretty sure the transformer will present an inductive load but what
about the total loading? What current is this transformer likely to draw
from the mains in order that it can supply 30W at 12V (that would be 2.5A @
12V, no?).

Is this timer switch likely to be sufficient or should I look for another
solution.

Thanks!

Gary wrote:
I've just purchased a set of 3 outdoor low voltage halogen lights to provide
some safety illumination for our somewhat gloomy path.
snip
The timer switch says it will switch 13A resistive or 5A inductive loads.
I'm pretty sure the transformer will present an inductive load but what
about the total loading? What current is this transformer likely to draw
from the mains in order that it can supply 30W at 12V (that would be 2.5A @
12V, no?).

Is this timer switch likely to be sufficient or should I look for another
solution.

240V/30W = .125A, call it .15A, so easily.

Gary wrote:
On Mon, 13 Nov 2006 17:14:38 +0000, Ian Stirling wrote
(in article ):

Gary wrote:
I've just purchased a set of 3 outdoor low voltage halogen lights to provide
some safety illumination for our somewhat gloomy path.
snip
The timer switch says it will switch 13A resistive or 5A inductive loads.
I'm pretty sure the transformer will present an inductive load but what
about the total loading? What current is this transformer likely to draw
from the mains in order that it can supply 30W at 12V (that would be 2.5A @
12V, no?).

Is this timer switch likely to be sufficient or should I look for another
solution.
240V/30W = .125A, call it .15A, so easily.

I thought about it after posting that 5A really is quite a lot and things
ought to be fine.

I suppose I'll find out tonight or tomorrow when I try them!


A loaded transformer isn't all that inductive. The warning is about
motors and choke-driven fluorescents. There will be a switch-on surge,
but largely due to the incandescent lamp filaments i.e. of the same
order as mains halogens. There's an additional surge with toroidal
transformers, but yours is unlikely to be one of those.

The problem with inductive loads is that turning them off in zero time
while current is flowing will generate an infinite voltage, so switches
will arc over to increase the turn-off time to the extent that the
voltage is just high enough to maintain the arc, if you see what I mean.

A heavy resistive load on a transformer (i.e. watts of a substantial
fraction of the rated VA for the transformer) will damp the process
considerably, dissipating the small amount of stored energy without
trying to keep it whizzing around an inductor.

Joe wrote:

A loaded transformer isn't all that inductive.

Agreed, but...

The warning is about motors and choke-driven fluorescents. There will
be a switch-on surge, but largely due to the incandescent lamp
filaments i.e. of the same order as mains halogens.

Not really, the impedance of the filaments is small compared with the
reactance of the choke, and it's the latter than determines the current
in the circuit. The filaments only drop about 6 V each, IIRC (Andrew
Gabriel will soon be along if I've got that wrong).

As with any circuit that involves a shunt winding across the supply -
transformer primary, motor stator, fluorescent choke, etc. - there may
be a transient at switch-on, as the magnetising current adjusts itself
into the correct phase relationship with the supply voltage. What this
does is to superimpose a unidirectional ("DC") transient on to the
steady-state current waveform. If the circuit is linear and the
inductance constant, the transient component of the current will decay
exponentially with time constant equal to L/R. However in practice the
circuit isn't linear because the transient current takes the iron core
into saturation, lowering the inductance and greatly increasing the
effect of the transient. Thus however resistive the circuit may look
under steady state conditions, there is always the scope for burning
switch or relay contacts at switch-on, or even welding them closed.

There's an additional surge with toroidal transformers, but yours is
unlikely to be one of those.

It's more prominent with toroidals, but affects any iron-cored
transformer (see above). Also many 12 V lighting transformers are in
fact toroidal - or used to be; most of them are electronic now. The
electronic ones have their own inrush current transient, charging the
reservoir capacitor.

The problem with inductive loads is that turning them off in zero time
while current is flowing will generate an infinite voltage, so switches
will arc over to increase the turn-off time to the extent that the
voltage is just high enough to maintain the arc, if you see what I mean.

The voltage would never be infinite, even without arcing, because stray
capacitance is always present in a real circuit and will limit the rate
of voltage rise.

A heavy resistive load on a transformer (i.e. watts of a substantial
fraction of the rated VA for the transformer) will damp the process
considerably, dissipating the small amount of stored energy without
trying to keep it whizzing around an inductor.

Yes, the time constant for the switch-on transient is determined by the
primary (magnetising) inductance and its winding resistance, whereas at
switch-off its the leakage inductance and load resistance (as seen on
the primary side) that matter.

--
Andy